Oxidation of a lubricating fluid can occur during ordinary, as well as severe, conditions of use. Oxidation causes the properties and the chemical structure of the fluid to change dramatically, leading to bearing corrosion, piston ring sticking, lacquer and sludge formation and excessive fluid viscosity. Current commercial synthetic lubricant base fluids are not suitable for very high temperature engine operation, they tend to degrade under rigorous thermal/oxidative conditions. Because of this, they have an operating ceiling of about 200.degree. to 250.degree. C., in the presence of antioxidants. While future engine temperatures increase to boost engine operating efficiency, new classes of base fluids need to be developed. Fluids such as polyphenyl ethers or perfluoro hydrocarbon fluids are useful at high temperatures; they are expensive and their lubricating properties are limited, particularly at low temperatures, having a pour point range of about 10.degree.-15.degree. C. which is not suitable for most lubricant uses. The common practice of the art utilizes an antioxidant to reduce the rate of oxidation of the lubricating oil and thereby improve the apparent thermal/oxidative stability.
Direct frictional contact between relatively moving surfaces even in the presence of a lubricant can cause wear of the surfaces. The elimination of wear is an ideal goal which is approached by blending the lubricating media with additives which can reduce the wear. The most suitable antiwear additives are those that help to create and maintain a persistent film of lubricant even under severe conditions such as high temperatures which thin the lubricant film and extreme pressures which squeeze the lubricant film away from the contacting surfaces. Wear is most serious in internal combustion engines, diesel engines and gasoline engines in which metal parts are exposed to sliding, rolling and other types of forceful, frictional mechanical contact. Specific areas of wear occur in the gears, particularly hypoid gears which are under high loads, piston rings and cylinders and bearings such as ball, sleeve and roller bearings.
Sulfur-containing derivatives, such as 4,4-thiodiphenol, are known to provide beneficial additive properties such as antioxidancy, antiwear and an ability to lubricate under extreme pressure conditions in lubricant applications. A concern, however, is that they are known to degrade upon exposure to high temperatures, i.e., over about 150.degree. C., during engine operation leading to engine corrosion and deposit formation.
Thiophenol-derived reaction products have been considered to solve these problems. For example, transition/alkali metal complexes of thiobis(alkylphenols) are described in U.S. Pat. No. 4,211,663 for their antioxidant performance in lubricating oils. However, for environmental and economic reasons it is desirable to avoid the use of metals in lubricant additives.
Thus, various non-metal derivatives have been developed. Borated diol-phenol sulfides are described in U.S. Pat. No. 4,906,390 for their friction and corrosion reducing properties in lubricants. U.S. Pat. No. 4,440,655 describes lubricant additives made by using a thiodiphenol in a Mannich reaction. U.S. Pat. No. 4,460,486 describes a polyoxyalkylene compound derived from 4,4'-thiodiphenol as useful as fiber and rubber lubricant additives having antioxidant properties. Derivatives of phenolic thioacetals as lubricant additives having antioxidant properties are described in U.S. Pat. No. 4,305,832.
A nucleophilic substitution reaction between alcohols and acylhalides to produce esters is known as described in March Advanced Organic Chemistry, pp. 346-347 (1985).